The presence of alkali metals in aluminum alloys, especially Al-Mg alloys, is known to cause cracking during sheet ingot rolling. This is particularly true of sodium for which concentrations as low as a few ppm can reduce formability under heated conditions (plasticity) and cause edge cracking during rolling operations. Hydrogen dissolved in molten aluminum can also cause problems during rolling as well as defects in the finished products.
It is known in the art to treat molten aluminum with chlorine gas and gaseous fluorine compounds. The chlorine gas is added to remove alkali and alkaline earth metals and hydrogen from the molten aluminum. The addition of the chlorine gas causes a brittle oxide crest to form on the metal. The fluorine gas, such as SF.sub.6, is added to prevent this oxide crust from forming. U.S. Pat. No. 5,145,514 (the disclosure of which is incorporated herein by reference) discloses that the fluorine and chlorine gases are introduced in a ratio of the fluorine gas to the chlorine gas of from 0.01 to 1.0. In addition, that patent discloses that the volume of the fluorine gas is between 2.12 and 10.75 NL/min. and the chlorine gas is between 0.24 and 4.45 NL/min.
U.S. Pat. No. 4,992,241 relates to a process for processing aluminum. The process includes treating molten aluminum with a combination of a nonreactive gas and a reactive gas. The reactive gas can be chlorine, SF.sub.6 or a mixture thereof. The reactive gas comprises between 2% and 20% by volume of the treatment gas. The patent discloses treatment gas flow rates in the range of 2-6 l/min. for a 300 kg melt.
U.S. Pat. No. 4,959,101 discloses a process for degassing aluminum melts without substantially altering the magnesium content of the melt. The process comprises contacting the melt with a mixture of an inert gas and a halogenated sulfur compound, such as sulfur hexafluoride. The sulfur compound is present in the inert gas in an amount between 2% and 20%. The amount of the treatment gas used is disclosed to be about 35 cubic feet per ton of aluminum at a pressure of about 30-45 psig.
U.S. Pat. No. 4,668,351 relates to a method of purifying aluminum contaminated with cerium or other rare earth metals. The method comprises contacting the molten aluminum with a halogenating agent, such as chlorine gas or aluminum fluoride. The amount of the halogenating agent is disclosed as being at least stoichiometric with respect to the cerium.
U.S. Pat. No. 4,138,245 relates to a process for removing sodium from an aluminum melt. The process comprises fluxing the molten aluminum with chlorine gas. The patent discloses a treatment rate of 1 liter per minute of chlorine gas for a 250 kg charge of aluminum.
It is believed that prior art utilizes relatively large chlorine flow rates, and, consequently, produces relatively high emissions and environmental pollution problems. Therefore, a need exists for an improved process for treating molten aluminum and aluminum alloys which utilizes lower amounts of chlorine and produces lower amounts of emissions without adversely affecting the cleanliness and quality of the treated metal.